U.S. patent application number 11/669034 was filed with the patent office on 2007-05-31 for method and apparatus of driving torsional resonance mode of a probe-based instrument.
Invention is credited to Robert C. Daniels, Chanmin Su.
Application Number | 20070119241 11/669034 |
Document ID | / |
Family ID | 36037010 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070119241 |
Kind Code |
A1 |
Su; Chanmin ; et
al. |
May 31, 2007 |
METHOD AND APPARATUS OF DRIVING TORSIONAL RESONANCE MODE OF A
PROBE-BASED INSTRUMENT
Abstract
A method of operating a scanning probe microscope includes using
a probe having a cantilever, and oscillating the probe at a
torsional resonance frequency thereof. In addition, the method
includes substantially increasing torsional drive efficiency with
dual actuators disposed on the probe or the probe base. First and
second actuators may be driven by corresponding first and second
drive signals, the first and second drive signals being about
180.degree. out of phase. The maximizing step includes altering at
least one of the amplitudes of the first and second drive signals
to maximize torsional oscillation. Torsional and flexural
oscillation of the cantilever probe can be excited concurrently,
sequentially or independently by adjusting the phase of the
corresponding drive signals. A pair of cantilever components can be
used to form a nanotweezer by rotating the respective arms having
corresponding tip portions at the distal ends.
Inventors: |
Su; Chanmin; (Ventura,
CA) ; Daniels; Robert C.; (Goleta, CA) |
Correspondence
Address: |
BOYLE FREDRICKSON NEWHOLM STEIN & GRATZ, S.C.
250 E. WISCONSIN AVENUE
SUITE 1030
MILWAUKEE
WI
53202
US
|
Family ID: |
36037010 |
Appl. No.: |
11/669034 |
Filed: |
January 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10937597 |
Sep 9, 2004 |
7168301 |
|
|
11669034 |
Jan 30, 2007 |
|
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10189108 |
Jul 2, 2002 |
6945099 |
|
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10937597 |
Sep 9, 2004 |
|
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Current U.S.
Class: |
73/105 |
Current CPC
Class: |
G01Q 60/38 20130101;
G01Q 60/32 20130101; G01Q 10/06 20130101; G01Q 60/26 20130101; G01Q
60/34 20130101; B82Y 35/00 20130101 |
Class at
Publication: |
073/105 |
International
Class: |
G12B 21/02 20060101
G12B021/02 |
Claims
1-41. (canceled)
42. A scanning probe microscope comprising: a probe having a tip; a
drive to oscillate said probe in torsion; and wherein said probe is
microfabricated so that the torsional oscillation of the probe is
effectively excited based on inertial forces caused by at least one
structural design characteristic of the probe.
43. The scanning probe microscope of claim 42, wherein said probe
has a longitudinal axis, and wherein said probe is asymmetric about
the axis.
44. The scanning probe microscope of claim 43, wherein the
asymmetry is defined by selectively positioning a tip of the probe
not on the axis.
45. The method of claim 43, wherein the probe includes a cantilever
along which the axis resides, and wherein the asymmetry is defined
by a shape of said cantilever.
46. The scanning probe microscope of claim 45, wherein the
cantilever includes more mass on one side of the centerline than on
the other side of the centerline.
47. The scanning probe microscope of claim 42, wherein a height of
the tip is selected so as to increase the inertial force of the
probe when driven into torsional oscillation.
48. The scanning probe microscope of claim 47, wherein the height
is greater than about 15 microns.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/189,108 filed on Jul. 2, 2002, the entirety
of which is expressly incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed operating a probe-based
instrument in torsional oscillation mode, and more particularly, a
method and apparatus of driving the probe into torsional
resonance.
[0004] 2. Description of Related Art
[0005] Several probe-based instruments monitor the interaction
between a cantilever-based probe and a sample to obtain information
concerning one or more characteristics of the sample. Scanning
probe microscopes (SPMs), such as the atomic force microscope
(AFM), are devices which typically use a sharp tip and low forces
to characterize the surface of a sample down to atomic dimensions.
More particularly, SPMs typically characterize the surfaces of such
small-scale sample features by monitoring the interaction between
the sample and the tip of the associated probe assembly. By
providing relative scanning movement between the tip and the
sample, surface characteristic data and other sample-dependent data
can be acquired over a particular region of the sample, and a
corresponding map of the sample can be generated. Note that "SPM"
and the acronyms for the specific types of SPMs, may be used herein
to refer to either the microscope apparatus, or the associated
technique, e.g., "scanning probe microscopy."
[0006] The atomic force microscope is a very popular type of SPM.
The probe of the typical AFM includes a very small cantilever which
is fixed to a support at its base and has a sharp probe tip
attached to the opposite, free end. The probe tip is brought very
near to or into direct or intermittent contact with a surface of
the sample to be examined, and the deflection of the cantilever in
response to the probe tip's interaction with the sample is measured
with an extremely sensitive deflection detector, often an optical
lever system such as described in Hansma et al. U.S. Pat. No. RE
34,489, or some other deflection detector such as an arrangement of
strain gauges, capacitance sensors, etc.
[0007] Preferably, the probe is scanned over a surface using a
high-resolution three axis scanner acting on the sample support
and/or the probe. The instrument is thus capable of creating
relative motion between the probe and the sample while measuring
the topography or some other property of the sample as described,
for example, in Hansma et al. U.S. Pat. No. RE 34,489; Elings et
al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No.
5,412,980.
[0008] AFMs may be designed to operate in a variety of modes,
including contact mode and oscillating flexural mode. In contact
mode operation, the microscope typically scans the tip across the
surface of the sample while keeping the force of the tip on the
surface of the sample generally constant by maintaining constant
deflection of the cantilever. This effect is accomplished by moving
either the sample or the probe assembly vertically to the surface
of the sample in response to sensed deflection of the cantilever as
the probe is scanned horizontally across the surface. In this way,
the data associated with this vertical motion can be stored and
then used to construct an image of the sample surface corresponding
to the sample characteristic being measured, e.g., surface
topography. Alternatively, some AFMs can at least selectively
operate in an oscillation "flexural mode" of operation in which the
cantilever oscillates generally about a fixed end. One popular
flexure mode of operation is the so-called TappingMode.TM. AFM
operation (TappingMode.TM. is a trademark of the present assignee).
In a TappingMode.TM. AFM, the cantilever probe is oscillated
flexurally at or near one of its resonant frequencies. When the tip
is in intermittent or proximate contact with surfaces the
oscillation amplitude will be determined by tip/surface
interactions. The amplitude or phase of this oscillation is kept
constant during scanning using feedback signals, which are
generated in response to tip-sample interaction. As in contact
mode, these feedback signals are then collected, stored, and used
as data to characterize the sample.
[0009] Independent of their mode of operation, AFMs can obtain
resolution down to the atomic level on a wide variety of insulating
or conductive surfaces in air, liquid or vacuum by using
piezoelectric scanners, optical lever deflection detectors, and
very small cantilevers typically fabricated using photolithographic
techniques. Because of their resolution and versatility, AFMs are
important measurement devices in many diverse fields ranging from
semiconductor manufacturing to biological research.
[0010] One limiting characteristic of AFMs and other probe-based
instruments lies in the above-described modes of operation. In an
AFM, the cantilever is typically oscillated using a piezoelectric
drive, often known simply as a piezo drive, to provide, for
example, a flexural oscillation mode. Referring to FIG. 1, a probe
assembly 20 includes a probe 21 having a cantilever 22 and a tip
28. The cantilever 22 extends outwardly from a base 24 of assembly
20. The cantilever 22 may be attached to the base 24 or formed
integrally with it. Base 24 is typically coupled to a piezoelectric
drive 26 (e.g., a piezo stack). Tip 28 is provided on the opposed,
free end of cantilever 22. Piezoelectric drive 26 can be
selectively excited by a signal generator 30 to move cantilever 22
up and down relative to a sample 32. When the instrument is
configured for flexural oscillation mode operation, the drive
voltage is applied to piezoelectric drive 26 to flexurally
oscillate the cantilever 22 about a lateral axis of the probe 21 at
a frequency that is dependent upon the frequency of the drive
voltage.
[0011] More particularly, in flexural oscillation mode, cantilever
22 is driven to resonate at its flexural resonance frequency or a
harmonic thereof about a lateral axis A-A' at the base 24 of
cantilever 22. Characteristics of cantilever flexural oscillation,
and changes thereof, are detected by quadrature photodetector 34,
typically with its vertical components, as shown by the arrow "V"
in FIG. 1. The deflection angle is sensed by photodetector 34 and
output as a voltage signal. Notably, the amplitude of the flexural
oscillation ranges between a few nm to 100 nm peak-to-peak
depending on the cantilever length.
[0012] In operation, as tip 28 approaches a surface of sample 32,
the flexural oscillation (tapping) amplitude starts to decrease due
to contact between tip 28 and sample 32. Notably, the flexural
vibration amplitude decreases to zero when tip 28 is pushed against
sample 32 with constant contact pressure. Variation of amplitude
between zero (generally continuous contact) and free oscillation is
typically used in a feedback configuration to control tip/surface
distance. Alternatively, the phase of the flexural oscillation may
be used to control this distance. Information relating to the
surface such as topology, hardness, and/or electromagnetic
properties is then determined by analyzing the signals that are
used to control this tip/surface spacing.
[0013] Overall, flexural oscillation mode AFMs are used to
characterize surface topology and surface energy dissipation by
monitoring the amplitude and/or phase of the oscillating
cantilever. This mode is often preferred to contact mode imaging
because it produces less damage to the tip and sample during
operation. However, operating the AFM based on flexural oscillation
of the cantilever is constrained in the following aspects.
[0014] Initially, flexural mode operation only detects surface
characteristics that impart a force in one direction, namely, the
vertical or "Z" direction. As a result, flexural mode AFMs do not
detect shear force interaction, and thus also cannot provide shear
force or force gradient information. This information is critical
to making measurements of surface friction, for example, when
attempting to identify surface compositional differences. When the
topography of the materials is generally undifferentiated, minimal
information is provided by flexural mode operation, and thus this
friction information becomes particularly valuable, and sometimes
necessary. Applications include identifying different components in
polymer blends, composites and other mixtures, identifying organic
and other contaminants on sample surfaces, delineating coverage by
deposited coatings and other surface layers, etc.
[0015] Moreover, without shear force or shear force gradient
measurement capabilities, flexural mode operation often results in
loss of other information relating to the sample. For example, when
a flexural oscillation mode AFM is used to image the magnetic
domain of a sample, only a force gradient in the direction
perpendicular to the sample surface can be sensed. Domains parallel
to the surface can only be seen at the domain boundaries where the
transitional region has a vertical force gradient. This limitation
also holds true for electric force imaging.
[0016] Other drawbacks associated with flexural resonance imaging
are slow kinetics and small amplitude errors that can drastically
limit scanning and data acquisition speed and compromise image
integrity. This effect is illustrated in the response curve 40 of
FIG. 2. In this case, A.sub.o is the free air amplitude of
oscillation (in RMS voltage), and A.sub.s is the set-point
amplitude for the control loop. When A.sub.o starts to decrease
from a constant value, A.sub.o, the tip starts to tap on the sample
surface. When tip/sample separation is reduced, and the tip and
sample interact, there is a corresponding change in the signal
produced by the deflection detection system. The amplitude of
flexural oscillation of the lever decreases due to it being
constrained by the sample surface as the tip approaches the surface
and taps the sample in each stroke of the oscillation. This is
shown in region "0" in which tip-surface distance .alpha.-axis) is
smaller than half of the peak-to-peak oscillation of the
cantilever. Notably, a feedback loop operates to move the
cantilever up and down to keep generally the same oscillation
amplitude A.sub.s. This movement reflects height changes in the
sample, i.e., surface topography.
[0017] The response of the cantilever in this flexural mode is
illustrated by the slope of the curve at region "O." In other
words, for a particular change in tip/sample separation
(.DELTA.Z.sub.1), the corresponding measured change in voltage
(.DELTA.V.sub.f) is relatively small given the shallow slope. It is
this measured change that determines the error that is processed by
the feedback loop to return operation to the set-point oscillation.
Because the slope of the cantilever response in flexural mode is
relatively shallow, scan speed must be kept small as relatively
large changes in tip-sample separation produce a relatively small
change in measured output, or error. Therefore, to facilitate
adequate data collection and integration of error signals, the scan
time at each location (or image pixel) must be long enough for the
system to respond with accuracy and resolution. The speed of data
acquisition must be correspondingly limited as well. An improvement
in data acquisition speed was desired.
[0018] Moreover, the shallow slope of the amplitude/distance curve
in FIG. 2 makes the control signal (voltage in the vertical axis)
correspond to a large height or distance compared to an
amplitude/distance curve with a steeper slope. As a result, the
control error will correspond to a greater quantity of height
measurement error. The situation is particularly problematic when
the probe is scanning across an abrupt step where slower response
due to error integration will result in even greater inaccuracy for
a given scan speed. Notably, such inaccuracy may be detrimental to
obtaining useful data in semiconductor metrology. The response of
an improved AFM, according to the present preferred embodiments
(AFM operating torsional resonance mode), is illustrated in FIG. 2A
and will described in further detail below.
[0019] Yet another limitation with flexural mode operation is that
the flexural resonance is very sensitive to the imaging environment
(e.g., when the sample is immersed in water), and thus oscillation
properties often change drastically, and in unpredictable ways,
upon change in imaging environment. Currently, the sensitivity of
flexural mode operation to imaging environment is one of the most
significant design considerations when configuring an AFM for
operation in fluid.
[0020] Other modes of AFM operation are similarly limited. For
example, shear force interaction between the probe in contact mode
and the corresponding sample surface has been studied with AFM for
a number of years. In an AFM technique known as lateral force
microscopy (LFM), the cantilever tip is dragged across the sample
surface, as in contact mode, to measure friction forces, as
described in U.S. Pat. No. 5,553,487 to the present assignee. More
particularly, using LFM, the tip is introduced to the sample
surface under a constant flexural deflection and then scanned along
the surface either in the direction of the cantilever length, or
perpendicular to the cantilever length. Using a laser-based
deflection detection system, the lateral cells of the corresponding
photodetector sense rotation of the cantilever as the tip of the
probe interacts with the sample through friction force. In the case
where tip scanning direction is perpendicular to the cantilever,
the difference of the lateral deflection during forward and reverse
scanning of the same portion of the sample is used as a relative
measure of the shear force, or surface friction. In addition to the
drawbacks associated with using contact mode to detect topology
characteristics, including tip/sample damage, etc., LFM suffers the
disadvantage of large tip/sample forces associated with contact
mode, and poor repeatability.
[0021] In other techniques, the tip placed in contact with the
sample surface is modulated by moving the sample surface laterally
relative to the probe. In this case, the lateral rocking of the
cantilever as a result of the contact friction is used to indicate
a quantity of surface friction. However, the lateral deflection
signals are small, and thus often unusable, and resolution is
insufficient for some of the applications contemplated by the
present invention.
[0022] In addition, although lateral deflection signals induced by
motion of the sample at acoustic frequencies can be enhanced, the
main control loop that defines tip/surface relative position still
employs vertical deflection (contact mode) feedback and, therefore,
suffers the drawbacks associated with flexural contact modes.
[0023] One challenge in implementing an AFM to image in torsional
resonance mode is that optimum performance depends on efficiently
driving the cantilever probe into torsional resonance. It is
important to note that typical AFM cantilever probes are
manufactured for flexural motion, i.e., motion in the "Z"
direction. This is primarily due to the fact that the optical
system used in detecting mechanical changes in the oscillating
motion of a cantilever are aligned in the "Z" direction. Also in
this regard, probes designed for flexural motion are easy to
produce in batches while preserving their planarity, which is
important to ensure accurate measurement of changes in flexural
oscillation.
[0024] A potential problem arises with standard AFM cantilevers
designed for flexural oscillation due to the fact that there is
physical asymmetry along the corresponding length of the lever
which although acceptable when driving the probe into flexural
oscillation, can render driving the cantilever into pure torsional
resonance difficult. For instance, if torsional resonance is
excited with two piezoelectric actuators driven out of phase, the
piezo-actuators must be disposed generally symmetrically about (1)
the length of the cantilever, along its central axis, and the (2)
corresponding tip to create pure rotational motion of the
cantilever about a rotation center, approximately equidistant from
the two piezoelectric actuators. The problem is, it is difficult to
insure that the central axis of the probe lies at the rotation
center of the drive. For instance, mounting the probe (i.e., probe
chip) in the AFM head is often an imprecise task due to allowable
variations in probe chip position, as well as structural variations
of the probe chip and probe itself. In addition, even if the
actuators are positioned equidistant from the central axis of the
cantilever, due to fabrication limitations, it is difficult to
determine whether the drive actuators are positioned symmetrically
about the tip which, although preferably resides at the central
axis of the lever, often does not.
[0025] Notably, in this regard, the tip of the probe attached to
the lever is relatively massive (it can be as much as fifteen
microns long) such that the inertia of the tip causes a torque.
This torque produces an arcing motion at the apex of the tip, but
given the scale that this motion occurs, the apex moves
substantially horizontally. Overall, however, it is difficult to
determine the amount of lateral motion that can be provided by such
a system due to inefficiencies coupling the energy to the tip
caused by the imperfect spatial relationship between the probe tip
and the drive.
[0026] As suggested previously, if the probe is well centered
between the two piezo actuators (eg., plates) that are employed to
drive the probe into torsional resonance, then the system will
excite pure lateral motion of the tip. As a practical matter,
however, because the tip typically is not centered, due to, for
example, imperfections produced during the probe manufacturing
process which may cause the tip to be positioned off the central
axis of the cantilever or the entire probe to be mounted off-center
intermediate the two piezo plates when secured in the AFM head.
[0027] Whatever the cause, this compromised relationship between
the drive and the probe can produce imperfect lateral motion of the
cantilever, and thus the tip. In particular, often times, this
motion will exhibit a vertical component. Such non-ideal motion
lowers the efficiency of operation in torsional resonance mode. For
instance, a vertical component in the cantilever motion can make
maintaining operation at the setpoint difficult. Moreover, due to
the high "Q" associated with torsional resonance mode, the overture
of the flexural vibration of the cantilever is fairly close to the
fundamental torsional resonance oscillation frequency of the
cantilever. Therefore, the vertical component of cantilever motion
may become mixed with torsional oscillation such that the system
becomes very unstable, with the possibility that AFM operation
toggles between, for instance, torsional resonance and flexural
resonance modes of AFM operation. Clearly, this unpredictability is
non-ideal. Overall, due to the many potential pitfalls with
insuring true alignment between the probe tip and the center of
rotation produced by the drive, including imperfect mounting of the
probe within the AFM had, an alternative driving arrangement was
desired.
[0028] Maintaining oscillation generally at the true torsional
resonance of the probe is particularly important when considering
the range of applications offered by operating the AFM in torsional
resonance mode. In an AFM application that is particularly
interesting, the probe is used to manipulate, for instance,
nanoparticles. Given the scale of operation, one key challenge in
using an AFM probe to perform nanomanipulation is determining
whether a particular operation associated with manipulating
nanoparticles has actually been accomplished. In standard AFM
operation, once a target to be manipulated has been identified and
an operation attempted by the AFM probe, there is no convenient way
to determine whether the target has actually been acted on. For
instance, if the operation is to pick up a target, one might think
that the change in weight at the tip could be measured. However,
because the target to be manipulated typically does not have an
appreciably greater weight than the tip, methods based on directly
measuring a change in weight are unreliable and, in any event,
difficult to implement. In fact, there is such a small change in
mass at the tip, e.g., one part in a million of the entire
cantilever, directly measuring the change is generally impossible.
As such, an alternate technique, preferably one which observes a
unique parameter associated with the AFM probe, was therefore
desired.
[0029] In addition, known techniques for performing
nanomanipulation, such as ones that employ what are known as
"nanotweezers," have significant limitations. Most such techniques
only have the ability to manipulate targets that are on the scale
of a micron or even a little larger. This is due to the fact that
there are often difficulties associated with locating the tweezers
at a location of interest, and controlling the force applied by the
tweezers to the sample, for example. In one technique,
electrostatic forces are used to actuate two adjacent columns or
beams. In this case, a voltage is applied to at least one of the
beams to modify the attractive force between the two beams, thus
causing the beams to close one against the other. By controlling
the voltage, the beams can be used to close on an object to be
manipulated. One problem with such a system is that large magnitude
forces are required to close the arms, which can compromise the
object being manipulated. Moreover, in this regard, the dimensions
of candidate target objects to be manipulated are correspondingly
limited in that objects that are too small cannot be feasibly
manipulated, especially given the higher voltages, and thus the
higher forces, that are difficult to accurately control.
Development of manipulation applications is continuing on the
nanoscale, and thus a superior design was desired. In particular,
an improved nanomanipulation device with the ability to close a gap
of about fifty to two hundred nanometers in a highly controllable
and accurate fashion, would be particularly valuable.
SUMMARY OF THE INVENTION
[0030] The preferred embodiments overcome the above-noted drawbacks
by aligning the rotational center produced by the torsional
resonance mode drive with the tip so that pure torsional motion can
be obtained. In the preferred embodiment, the power provided by the
two piezos driving the cantilever into torsional resonance is
generated by applying signals out of phase with each other and
having varying amplitudes so as to distribute power such that the
rotational center of the motion directly overlies the apex of the
tip. More particularly, by appropriately modifying the amplitudes
of the signals applied to the piezo plates, the rotational center
of the cantilever can be moved to produce ideal torsional motion
and optimum data acquisition capabilities in torsional resonance
(TR) mode.
[0031] According to a first aspect of the preferred embodiment, a
method of operating a scanning probe microscope includes using a
probe having a cantilever and oscillating the probe at a torsional
resonance frequency thereof. In addition, the method includes
substantially maximizing an amplitude of probe oscillation.
[0032] In another aspect of this embodiment, the oscillating step
includes using first and second actuators disposed relative to the
probe so as to generate torsional motion of a tip of the probe.
Preferably, the actuators are piezoelectric actuators.
[0033] In a still further aspect of this embodiment, the first and
second actuators are driven by corresponding first and second drive
signals, the first and second drive signals being about 1808 out of
phase. The maximizing step includes altering at least one of the
amplitudes of the first and second drive signals.
[0034] According to another aspect of this embodiment, the altering
step includes using a balance mode control unit to adjust the
amplitudes of the first and second drive signals so that the
torsional response at a given drive power is maximized. The balance
control unit preferably includes a voltage multiplier and operates
automatically to substantially maximize the torsional response
amplitude.
[0035] According to a further aspect of this preferred embodiment,
the probe is an active probe having at least one active element and
at least one corresponding electrode integrated with the probe.
Ideally, the probe includes first and second electrodes adapted to
receive corresponding first and second drive signals.
[0036] According to yet anther aspect of this preferred embodiment,
the cantilever is split into first and second longitudinal portions
and a tip at a distal end of the cantilever is split into first and
second tip portions that are actuatable so as to form a
nanotweezers.
[0037] In accordance with another aspect of this preferred
embodiment, a method of operating a scanning probe microscope in
torsional resonance mode includes oscillating a tip of a probe with
an actuator at a torsional resonance frequency of the probe.
Preferably, the oscillating step includes substantially maintaining
an axis of rotation provided by the actuator so that it
substantially directly overlies an apex of the tip.
[0038] According to a further aspect of this preferred embodiment,
the maintaining step includes balancing the output of the actuator.
The actuator preferably includes first and second actuator elements
disposed generally symmetrically about the probe and the balancing
step includes applying first and second drive signals to the first
and second actuators. The applying step includes selecting the
amplitudes of the first and second drive signals having opposite
phase automatically.
[0039] In yet another aspect of the preferred embodiment, the
oscillating step is performed using a shear piezoelectric actuator.
Moreover, the vertical and shear actuators are preferably
coupled.
[0040] According to a still further aspect of the preferred
embodiment, a scanning probe microscope includes a probe having a
tip, and a drive to oscillate the probe at a torsional resonance of
the probe. The drive preferably has an axis of rotation that
substantially directly overlies an apex of the tip.
[0041] In another aspect of this embodiment, the drive includes a
drive source and an actuator including first and second actuator
elements. The drive source applies first and second drive signals
to the actuator elements.
[0042] In a still further aspect of this embodiment, a mode control
unit is employed to balance the drive so that the torsional
oscillation of the probe is substantially maximized. Preferably,
the mode control unit includes a voltage divider.
[0043] These and other objects, features, and advantages of the
invention will become apparent to those skilled in the art from the
following detailed description and the accompanying drawings. It
should be understood, however, that the detailed description and
specific examples, while indicating preferred embodiments of the
present invention, are given by way of illustration and not of
limitation. Many changes and modifications may be made within the
scope of the present invention without departing from the spirit
thereof, and the invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] A preferred exemplary embodiment of the invention is
illustrated in the accompanying drawings in which like reference
numerals represent like parts throughout, and in which:
[0045] FIG. 1 is a schematic view of a prior art scanning probe
microscope configured for flexural mode operation, appropriately
labeled "Prior Art";
[0046] FIG. 2 is a curve illustrating the response of the flexural
mode AFM shown in FIG. 1, appropriately labeled "Prior Art";
[0047] FIG. 2A is a curve illustrating the response of a torsional
resonance mode AFM, as shown for example in FIG. 9;
[0048] FIG. 3 is a front elevational view of a probe assembly
including a piezoelectric actuator and a probe chip mounted in a
probe holder;
[0049] FIG. 4 is a view similar to FIG. 3, illustrating using two
piezoelectric actuators to excite a torsional resonance of the
probe;
[0050] FIG. 5 is a front elevational view similar to FIGS. 3 and 4,
illustrating balancing the output of the piezoelectric actuators to
excite pure torsional resonance, according to the preferred
embodiment;
[0051] FIG. 6 is a graph showing torsional amplitude versus
frequency, illustrating a peak response amplitude of the probe
oscillating in torsion;
[0052] FIG. 7 is a perspective view of a probe assembly including
two actuators for exciting torsional resonance where the actuators
are disposed generally symmetrically about the central axis of the
cantilever;
[0053] FIG. 8 is a perspective view similar to FIG. 7, with the
rotation center of the actuators generally not overlying the center
longitudinal axis of the cantilever of the probe, illustrating
drive signals applied to the piezoelectric actuators having
different amplitudes to balance the drive;
[0054] FIG. 9 is a schematic diagram illustrating an atomic force
microscope employing a mode control unit to drive a probe into
torsional oscillation, according to a preferred embodiment;
[0055] FIG. 10 is a schematic diagram of the mode control unit of
FIG. 9;
[0056] FIG. 11 is a schematic diagram of an atomic force microscope
according to a preferred embodiment, including a probe assembly,
the mode control unit, and a feedback loop;
[0057] FIG. 12 is a perspective view of a probe usable in flexural
and torsional modes, illustrating integrated actuator elements;
[0058] FIG. 13 is perspective view of a probe including integrated
actuator elements similar to FIG. 12, according to an alternate
preferred embodiment;
[0059] FIG. 14 is a perspective view of a probe with a split
cantilever similar to that shown in FIG. 12, illustrating two pairs
of electrodes for exciting both flexural and torsional
oscillation;
[0060] FIG. 14A is a front elevational view of a nanotweezer tip of
the probe;
[0061] FIG. 15 is a perspective view illustrating an alternate
arrangement for exciting torsional resonance of a probe, including
a piezoelectric actuator for flexurally oscillating the probe;
[0062] FIG. 16 is a schematic graph illustrating amplitude versus
frequency, illustrating the response of the probe in flexural mode
including using the probe shown in FIG. 13 and the control system
shown in FIG. 9; and
[0063] FIG. 17 is a schematic graph similar to FIG. 16,
illustrating the response of the probe in torsional or lateral
mode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0064] A probe-based instrument, such as an AFM, that analyzes one
or more characteristics of a sample in a torsional resonance mode
is shown in the Figures and described herein. More particularly, a
number of techniques for generating a steady torsional resonance in
the cantilever of the SPM probe assembly is discussed. TR mode,
together with feedback control, is used to image or otherwise
characterize sample surfaces.
[0065] During operation, a property of a torsional oscillation of
the probe (e.g., amplitude or phase of probe oscillation about the
longitudinal centerline of the probe's cantilever) is monitored to
determine sample surface characteristics. Notably, the torsional
signal detection and feedback control is preferably implemented
using configurations similar to that used in known AFMs, such as in
the AFMs offered by the Veeco Instruments Inc. However, compared to
known AFM operating modes, including flexural oscillation mode
operation, the potential benefits of the present torsional
resonance mode of AFM operation include improved scanning speed, as
well as offering the ability to significantly improve small-scale
mass detection and nanomanipulation of particles. Moreover, these
benefits can be achieved while simultaneously operating the AFM in
conventional flexural mode.
[0066] Note that when the term "horizontal motion" is used herein,
it is intended to refer to the motion that is produced by the
torque that rotates the tip so that the apex of the tip moves
parallel to the sample surface. However, because the scale of the
arc that is traced out by the motion of the tip is relatively
small, motion is effectively only in the horizontal direction.
Modes of Operation
[0067] With reference to FIG. 3, a probe assembly 50 of a
conventional atomic force microscope (not shown) includes a probe
chip 52 having a probe 54 defining a tip 56. The probe chip 52 is
mounted within a probe pocket 58 of a probe holder 60 that supports
the probe chip 52. To excite the probe 54 into resonance, an
actuator 62 (for example, a piezoelectric actuator such as a piezo
stack) is mounted to the probe holder 60 and is used to drive the
probe according to the mode of operation. Notably, probe 54 can be
driven into flexural oscillation or torsional oscillation.
[0068] Piezoelectric actuator 62 is typically a vertical actuator
that oscillates the probe 54 into flexural motion. When driven in
this fashion, the tip 56 of probe 54 will tap on the surface of the
sample under test (not shown) as the flexural motion of the probe
54 is monitored, for example, with a laser detection scheme. By
employing a vertical piezoelectric actuator, lateral motion of the
probe 54 cannot be achieved and thus torsional resonance cannot be
excited.
[0069] Turning to FIG. 4, a probe assembly 70 for exciting a
torsional resonance of a probe 74 of a probe chip 72 includes a
drive 77 having an actuator 78, preferably defining a pair of
piezoelectric elements 80, 82, such as piezo stack actuators
mounted generally symmetrically about the center of the probe
holder 60. Again, probe holder 60 includes a probe pocket 58
configured to receive probe chip 72. In this case, when driving
actuators 80, 82 with signals i.sub.1 and i.sub.2 of generally
equal amplitude but opposite phase, a tip 76 of probe 74 of probe
chip 72 moves laterally as the system attempts to achieve
oscillation of the probe 74 at a torsional resonance of the probe,
as described in detail in the application expressly incorporated by
reference herein. Note that the drive source(s) that produce
i.sub.1 and i.sub.2 (not shown in FIG. 4), together with actuator
78, may hereinafter be referred to collectively as the drive
77.
[0070] One problem, however, is that the actuator 78 often times is
not disposed symmetrically about probe 74, and particularly the tip
76, when probe chip 72 is mounted in probe holder 60. As described
in detail in the Background, this causes non-ideal torsional
oscillation of the probe.
[0071] More particularly, tip 76 of probe 74 often times does not
lie on a line through the center of the entire probe chip 72
(marked with a dashed line labeled "C"), perpendicular to the
backside of the cantilever of probe 74, as shown in FIG. 4, and
thus this lack of alignment yields compromised lateral oscillation.
Although the probe chip 72 typically can be mounted generally
symmetrically relative to elements 80, 82, the tip 76 of probe 74
often does not lie at the center point between actuator elements
80, 82, which generally define the center of rotation of actuator
78 (including actuator pair 80 and 82), marked "C". As a result,
energy provided by actuator 78 coupled to the off-center tip 76
will be unequal from either side if drive signals i.sub.1 and
i.sub.2 have equal amplitudes and opposite phase. Therefore, pure
torsional resonance will not predictably be achieved and the
performance of the AFM in TR mode will be correspondingly
compromised. In fact, pure torsional resonance typically will not
be achieved unless the tip 76 of probe 74, by mere fortuitous
chance, directly underlies the center of rotation of actuator 78.
Again, this most often will not be the case due to the
nonpredictability of the manufacturing and mounting processes.
[0072] To overcome this non-alignment problem between tip 76 and
the center of rotation of actuator 78, a preferred modification to
the assembly 70 shown in FIG. 4 has been made and is shown as
assembly 90 in FIG. 5. Turning to FIG. 5, probe assembly 90
includes a drive 92 having a drive source (not shown) that applies
drive signals i.sub.3 and i.sub.4 to actuator 78. Although
similarly out of phase, i.sub.3 and i.sub.4 may be applied at
different amplitudes, contrary to drive signals i.sub.1 and i.sub.2
that have equal amplitudes. In this case, the virtual rotation
center created by piezo pair 80 and 82 is shifted from C to C',
substantially directly overlaying the top of the cantilever probe
rotation axis. Notably, such a shift will reduce flexural component
of the drive acting upon the cantilever probe.
[0073] As shown in FIG. 5, drive signal i.sub.3 is applied with a
greater amplitude than i.sub.4. As a result, the center or axis of
rotation provided by actuator 78 is shifted from a line "C" about
which the actuator elements 80, 82, are disposed symmetrically to a
line marked "C'" that is substantially centered or aligned with the
actual position of tip 76. Because, in this case, tip 76 is
disposed right of the center line defined by actuator elements 80,
82, as mounted within probe holder 60, the amplitude of the signal
applied to the right actuator element 82, namely i.sub.4, is
applied with a smaller amplitude than i.sub.3.
[0074] It is notable that it is difficult to predict exactly where
tip 76 lies relative to actuator 78. Therefore, the values of the
amplitudes of i.sub.3 and i.sub.4 are selected, either manually or
automatically, to achieve the greatest response amplitude. The
output of this selecting operation is illustrated by "A.sub.1" and
"A.sub.2" in FIG. 6.
[0075] More particularly, as the amplitudes of the drive signals
i.sub.3 and i.sub.4 are modified, the amplitude of the response of
probe 74 oscillating at its torsional resonance frequency is
observed. This is shown, for example, in FIG. 6. By driving the
probe with a balanced drive (i.e., response "A.sub.2" in FIG. 6),
efficient coupling of the drive energy to tip 76 for torsional
oscillation is provided and the response is optimized at a
torsional resonance frequency of probe 74 to achieve an optimum
torsional response thereof.
[0076] In one preferred embodiment, this may be accomplished by
trial and error as the amplitudes of the two signals are modified
and the response of probe 74 is monitored by a user. As the i.sub.3
and i.sub.4 amplitudes are adjusted, the torsional amplitude can be
made to shift from A.sub.1 to A.sub.2 (FIG. 6) at the resonance
frequency. Preferably, this function is implemented with a balance
control circuit, described in further detail below in connection
with FIGS. 9 and 10, that controls the drive signal applied to each
actuator element 80, 82. Again, effectively, the rotation center
provided by actuator elements 80, 82, upon application of
corresponding signals i.sub.3 and i.sub.4, causes the center of
rotation provided by the elements, in the case shown in FIG. 5, to
move to the right so that the center of rotation substantially
directly overlies an axis along the length of the cantilever and
through tip 76 of probe 74, in other words, so the axis of rotation
provided by the drive is substantially aligned with the probe
tip.
[0077] This operation is illustrated similarly in the out of scale
perspective views of FIGS. 7 and 8. As shown in FIG. 7, a probe
assembly or chip 100 includes a probe 102 defining a tip 104
supported by a cantilever 106. In this case, a driving mechanism
108 is supported by probe chip 100 itself, rather than a probe
holder, as in the embodiments illustrated in FIGS. 3-5. When probe
102, and in particular tip 104 of probe 102, is disposed at a plane
generally at the geometric center between a pair of actuator
elements 110, 112 of driving mechanism 108, a pure torsional
resonance may be achieved by applying signals having equal
amplitudes, A.sub.0, but opposite phase. However, for the probe
assembly 120 shown in FIG. 8, where tip 124 of probe 122 is not
situated at the rotational center of a driving mechanism 128, i.e.,
not equidistant from actuator elements 130, 132 of driving
mechanism 128, exciting the actuators with signals having equal
amplitude but opposite phase will not yield the optimum torsional
resonance response. In fact, the torsional amplitude will be as
generally shown with the solid line in FIG. 6.
[0078] To achieve optimum response at the torsional resonance
frequency, input signals having the opposite phase and different
amplitudes (A.sub.1 and A.sub.2) are applied to the actuator
elements 130, 132. In this case, because probe 122 is situated
closer to actuator element 130 than element 132, A.sub.2 should be
greater than A.sub.1 upon balancing the drive 128, to move the
rotational center thereof. Again, balancing the drive is
accomplished by the user manually, via observation, or it can be
implemented automatically using a control algorithm, as described
in further detail below. Again, as discussed previously, by
balancing the drive, the rotation center of actuator 128 is shifted
from a position on a center plane between the two actuator elements
130, 132 to a position that is substantially contiguous with the
center line of probe 122 through tip 124. As a result, pure
torsional resonance is achieved, thus maximizing sensitivity and
otherwise optimizing performance.
[0079] Turning next to FIG. 9, an AFM 150 operating in torsional
resonance mode includes a circuit for maintaining optimum
oscillation of the probe at a torsional resonance thereof. In
particular, a probe assembly 152 including a probe 154 mounted to a
probe holder (not shown) that supports a drive mechanism 156
including a pair of piezoelectric actuator elements 158, 160 is
shown. To drive piezoelectric actuators 158, 160, a mode control
unit 162 coupled to a piezoelectric drive source 164 (i.e., an
oscillating voltage source) processes the drive signal to generate
signals labeled "Piezo Drive 1" and "Piezo Drive 2" in FIG. 9,
which feed to actuators 158, 160, respectively. A balance control
block 166 is provided to control the output of mode control unit
162 so that drive signals having appropriate amplitudes, namely
"Piezo Drive 1" and "Piezo Drive 2", excite pure lateral motion of
the tip at a torsional resonance of the probe, as described in
further detail below in connection with FIG. 10.
[0080] In operation, probe 154 is driven into oscillation while a
detection system 170 monitors probe oscillation, including changes
thereof. Detection system 170 includes a light source (not shown)
that generates a laser beam "L", for instance, that is directed
towards a back side 174 of a cantilever 172 of probe 154, such that
the beam is reflected off cantilever 172 and towards a sensor 178.
Preferably, sensor 178 is a quadrature photodetector that can
monitor both lateral changes and flexural changes in the
oscillation of probe 154, as is well known in atomic force
microscopy. Signals from sensor 178 are then transmitted to the
mode control unit 162 whereby changes in the reflected lateral AC
signal and the reflected vertical AC signal can be discriminated.
More particularly, depending upon the mode of operation, either
flexural mode or torsional mode, the lateral AC signal or the
vertical AC signal is monitored to detect changes from the
set-point characteristic of oscillation (for example, amplitude or
phase) in response to interaction between a tip 176 of probe 154
and the sample (not shown). Using a feedback controller 168, the
set-point can be maintained based on the mode of operation in
conventional fashion. These changes can then be plotted to map
sample characteristic(s) under test.
[0081] Again, critical to optimum operation is balancing the drive
to achieve pure torsional resonance of probe 154. In this regard,
mode control unit 112 is employed to appropriately drive probe 154
at torsional resonance when in torsional resonance mode, and drive
probe 154 into flexural resonance in vertical oscillating mode.
Turning to FIG. 10, mode control unit 162 is shown in full detail.
Mode control unit 162 includes three switches including two state
switches 180, 182 and a signal switch 184. State switches 180, 182
operate together with signal switch 184 to coordinate the
excitation signals applied to piezoelectric actuator elements 158,
160, as well as discriminate which signals (either torsional or
flexural) acquired by the detection system 170 (FIG. 9) should be
further processed.
[0082] More particularly, in flexural mode, state switches 180, 182
operate to transmit the signal applied to their top inputs, which
in each case is the output of the piezo drive source 164 directly,
with no further signal processing. As a result, the same signal,
including phase and amplitude, is applied to the piezoelectric
actuators of the drive so that the flexural resonance of the probe
154 (FIG. 9) can be excited. In this mode, signal switch 184 is set
so that it passes the vertical AC signal obtained from sensor 178
(FIG. 9) to controller 168. Controller 168 is preferably a standard
SPM controller that receives the signal indicative of oscillation
of probe 154 (in either torsional or flexural mode) and operates
using feedback to maintain the set-point oscillation, as noted
above.
[0083] When the state switches 180, 182 are set for torsional mode
operation, the signals applied to the piezoelectric actuator
elements can be modified. A voltage multiplier (e.g., a voltage
divider 186) is provided to modify (in this case split) the voltage
applied by piezo drive 164 to mode control unit 162 according to
the setting of balance control unit 166. More particularly, in this
case, the divided voltage is applied to operational amplifiers 188,
190 (i.e., differential amplifiers, FIG. 10) which generate a
.DELTA.V.sub.1 and a .DELTA.V.sub.2 that are used to drive the
piezoelectric actuators. Moreover, an inverter 192 is included to
condition the .DELTA.V.sub.2 signal 180 degrees so that
.DELTA.V.sub.1 and .DELTA.V.sub.2 are opposite in phase. In sum,
with state switches 180, 182 set for torsional resonance mode
operation, signals having appropriate amplitudes and being 180
degrees out of phase are applied to the piezoelectric actuators to
excite pure torsional resonance oscillation of tip 176 of probe 154
(FIG. 9).
[0084] Importantly, balance control unit 166 may be a manually
operated knob (operable similarly as a left-right balance control
knob of an audio amplifier), or it may be implemented with
appropriate electronics in feedback to automatically set the
balance control, and thus the applied signals "Piezo Drive 1" and
"Piezo Drive 2," to achieve the maximum output amplitude, as
illustrated in FIG. 6. When manually selecting the amplitude of the
signals to balance the drive, the operator observes, in real-time,
the response of the probe's oscillation. Once the rotation center
of the two actuators is aligned with the tip of the probe, a
maximum amplitude response of the torsional oscillation of the
probe will occur and can be observed by the user at which time the
balance control is set.
[0085] Alternatively, this balancing is done automatically using
appropriate circuitry and control software. Specifically, an
algorithm can be employed to evaluate the response of the probe
(i.e., lateral AC input (e.g., amplitude) to mode control unit 162)
and, based on the response, appropriately adjust the balance
control unit 166 so that "Piezo Drive 1" and "Piezo Drive 2" cause
the probe to oscillate with a maximum amplitude at torsional
resonance. Notably, in torsional resonance mode, signal switch 184
may be set so that the lateral AC signal acquired by the sensor is
transmitted from mode control unit 162 to controller 168 to
maintain probe oscillation at the set-point.
[0086] Next, turning to FIG. 11, operation of AFM 150 is shown in
further detail. Generally, mode control unit 162 is used to
maintain oscillation of tip 162 of probe 154 at its torsional
resonance while a feedback loop is employed to control tip-sample
separation by maintaining a characteristic of the oscillation of
probe 154 at a set-point. The set-point may be either a flexural
mode set-point or a torsional resonance mode set-point. In
operation, sensor 178 receives the laser beam reflected from
backside 174 of cantilever 172 of probe 154 and transmits, for
example, a lateral deflection signal to a signal processing unit
200 for ultimate comparison to the TR Mode set-point. In
particular, signal processing unit 200 receives an output signal
from sensor 178 and conditions that signal so that it may be
compared to the set-point. Signal processing unit 200 may be an
RMS-to-DC converter that generates a lateral RMS signal that is
transmitted to, for instance, a comparator 202. An error signal
based on a comparison of the set-point with the lateral RMS signal
is generated and transmitted to a control block 204 (e.g., a PI
controller) of the feedback loop. Controller 204 applies an
appropriate gain to the error signal to generate a control signal
(labeled "Z Drive") that may be used to control the Z position
drive actuator 206 (for example, the Z section of a piezoelectric
XYZ scanner) to control tip-sample separation. In this case,
actuator 206 translates probe assembly 152 coupled thereto so that
the selected characteristic of probe oscillation returns to the TR
mode set-point.
[0087] Alternatively, the system may be operated in flexural mode.
In this case, rather than a lateral deflection signal transmitted
to signal processing unit 200 by the quadrature photodetector 178,
a probe deflection (flexural) signal is transmitted to signal
processing unit 200. Notably, in this regard, optional signal
switch 184 of mode control unit 162 shown in FIG. 10 is not
activated, as shown with the dashed lines in FIG. 11.
[0088] Turning next to FIG. 12, a probe assembly 210 according to
an alternate preferred embodiment of achieving optimum torsional
oscillation implementing an active lever such as that shown and
described, e.g., in co-owned U.S. Pat. Nos. 6,189,374 and 6,530,266
is illustrated. Probe assembly 210 is similar to the
above-described probe assembly/chip in that two actuator elements
are used to drive the tip into torsional resonance, but the
actuator elements are integrated with the probe to generate this
torsional motion. Because the actuation layer is fabricated with
the cantilever in a batch process, the torsional rotation axis of
the actuator pair substantially overlaps the rotation axis of the
cantilever composite. Actuation could be a pure torque, providing
the cleanest way of driving torsional oscillation mode. More
particularly, probe chip 210 includes a base 212 and a probe 214.
Probe 214 defines a cantilever 216 extending from base 212, the
cantilever supporting a tip 218 and which may be oscillated in both
flexural mode and in torsional resonance mode.
[0089] A pair of actuator elements 220, 222 are preferably formed
with the probe and comprise a piezoelectric material such as zinc
oxide. Base 212 and cantilever 216 of probe chip define a top
surface upon which the first and second zinc oxide actuator
elements 220, 222 are disposed generally symmetrically about a
longitudinal plane passing through tip 218 of probe 214,
perpendicular to the top surface of cantilever 216. On top of each
of the zinc oxide actuator elements 220, 222 is disposed a
respective electrode 224, 226 via which the piezo drive signals are
applied to the elements. In this arrangement, the energy coupled to
tip 218 of probe 214 does not need to propagate through additional
structure (such as probe holder 60 in FIG. 5 or base 109 in FIG. 7)
as it does in the above-described embodiments, and thus the energy
is coupled to tip 218 more efficiently, enhancing this probe's
ability to achieve oscillation at pure torsional resonance.
[0090] Notably, in this embodiment, a central portion 228 of
cantilever 216 of probe 214 can be machined so that the probe
defines two separate parallel arms 230, 232 joining together again
at the point 229. The zinc oxide actuator elements 220, 222 and
corresponding electrodes 224, 226 are disposed upon the two
separate arms 230, 232, thus reducing the lateral stiffness of
cantilever 216. Torsional stiffness thus approaches the flexural
stiffness of the probe which can permit more predictable excitation
of torsional oscillation. In addition, when using the mode control
unit 162 (FIGS. 9-11), tip motion can be efficiently controlled
such that flexural response and torsional response can be
completely separated by resonance frequency range.
[0091] Turning to FIG. 13, an alternative embodiment of using an
active lever with split actuators usable in torsional resonance
mode is shown. A probe assembly 250 includes a base 252 and a probe
254 defining a cantilever 256 extending from base 252. Cantilever
256 supports a tip 258 and has a backside 260 supporting integrated
actuator elements 262, 264, preferably zinc oxide elements. A pair
of respective electrodes 266, 268 is disposed on actuator elements
262, 264 to allow actuators 262, 264 to receive appropriate drive
signals. Functionally, the FIG. 13 probe chip 250 is generally
identical to the assembly 210 shown in FIG. 12, however it is
easier to fabricate. This is primarily due to the fact that two
parallel cantilever arms are not formed by machining the cantilever
through a central axis thereof. Notwithstanding ease of
fabrication, however, the torsional stiffness of probe chip 230 is
higher than probe chip 210 shown in FIG. 12, and coupling between
torsional motion and flexural motion may occur, particularly in the
subresonance frequency region. Such coupling is unacceptable and
must be monitored and accounted for, for example, by proper control
of tip oscillation at resonance.
[0092] In FIG. 14, another alternate design of a probe chip 270 is
shown. In this case, a cantilever 274 of a probe 272 is split into
two branches 276, 278 that are capable of rotating independently,
as with probe 214 shown in FIG. 12. Similar to probe 214, probe 272
has an opening 280 along a length or central axis of cantilever
274, for example, machined therein, to define arms 276, 278. Arms
276, 278 are fixed to a base 271 of assembly 270 on one end, while
the opposite ends of arms 276, 278 are coupled at a distal end 282
of probe 272 to support a tip 284. Moreover, the width of branches
276, 278 at region 286 of cantilever 274 in this case is slightly
larger than in the split cantilever probe chip 210 shown in FIG. 12
near base 271 to accommodate two electrodes 288, 290, 292, 294 on
each branch and allow flexural oscillation of tip generally about
axis A-A'. Line O-O' represents a fine cut 280 through the
cantilever probe 272, preferably using a focused ion beam, thus
creating a cross-section profile at B-B' as seen in FIG. 14A. As
the two arms 272 and 286 rotate in opposite directions, the apex
portion of probe tip 284 will open and close, serving as a
nanotweezer with generally two to three orders of magnitude larger
force than a typical electrostatic force. In this embodiment, one
pair of the electrodes 288, 290, for instance, on the two branches
of cantilever 274 is used to excite flexural oscillation of tip
284, while the opposite pair of electrodes 292, 294 are used to
selectively rotate tip portions 285, 287. Notably, if flexural mode
is desired, the tip portions 285, 287 are rotated into engagement
with one another.
[0093] An alternate preferred method of exciting torsional
resonance of a probe in an SPM application is shown in FIG. 15.
More particularly, a probe assembly 300 including a driving
mechanism 302 coupled to a base 304 of probe assembly 300 is
operable at both a flexural and a torsional resonance of a
corresponding probe 306. A vertical actuator 308 driven by a
voltage V.sub.v is preferably coupled to a shear mode actuator 310
(e.g., a piezo stack) driven by a voltage V.sub.T such that either
a flexural resonance or a torsional resonance of probe 306 can be
excited, via inertial force of the tip for torsional resonance and
inertial force of the cantilever for flexural resonance. In this
example, shear mode piezo 310 provides lateral movement that is
coupled to probe 306 and ultimately a tip 307 of probe 306 to
oscillate the tip in torsion. The user separately controls V.sub.v
and V.sub.T to excite either vertical resonance or torsional
resonance as preferred. Optionally, an insulated spacer 312 may be
provided between the vertical piezo and the shear mode piezo to
isolate and optimize coupling of the corresponding energy to the
probe tip 307.
[0094] Notably, unlike the previously described embodiments,
balance control is not needed to optimally drive probe tip 307 at
torsional resonance. This is due to the fact that shear piezo
actuator 310 is employed to directly facilitate lateral motion of
tip 307, i.e., the problem associated with the center of rotation
of the driving mechanism does not exist. Overall, by mounting
piezoelectric elements 308, 310 on a base 304 of probe assembly 300
as shown, an image of a sample in flexural mode can be generated
while nearly immediately being able to image the same scan line of
the sample in torsional resonance mode with appropriate control of
V.sub.v and V.sub.T.
[0095] An illustration of the response of probe 230 shown in FIG.
13 as operated in AFM system 150 in FIG. 11 is illustrated in FIGS.
16 and 17. More particularly, in FIG. 16, once switched to operate
in flexural mode, the vertical signal output from the quadrature
photodetector (for example, 178 in FIG. 11) shows peaks at
resonance (about 50, 200 and 366 KHz). At the peaks of the plotted
signal in flexural mode, the corresponding lateral signal is shown
with an ideally small amplitude, as desired. Once switched to
torsional resonance mode, with the lateral drive, the lateral AC
signal detected shows peaks at a resonance frequency of about 175
kHz and at an overtone at about 590 kHz, as shown in FIG. 17.
Similar to flexural mode, the vertical signal at the frequencies
associated with the peaks of the torsional response is
correspondingly small, as desired.
[0096] Referring back to FIGS. 2 and 2A, an illustration of the
response 42 achieved by an AFM operating in torsional resonance
(TR) mode according to the present preferred embodiments, as well
as the flexural mode response (FIG. 2), is shown. In FIG. 2, the
amplitude response of an AFM operating in conventional flexural
oscillation mode to a change in tip-sample separation, .DELTA.Z, is
a illustrated as a change in the detected voltage signal,
.DELTA.V.sub.f. Notably, the corresponding slope,
.DELTA.V.sub.f/.DELTA.Z, is indicative of the response and defines
data acquisition speed and accuracy. The larger the slope, the more
sensitive the AFM is to small changes in tip-sample interaction,
and the faster the AFM image can be obtained. This is due in part
to minimizing electrical noise, given the inherent physics
associated with oscillating the lever in torsion. In addition, the
feedback controller (e.g., a PI loop) is responsive to the
magnitude of the feedback error signal generated during operation.
Because much smaller changes in tip-sample separation create much
larger changes in the amplitude response (error signal is larger),
data may be acquired significantly faster when the AFM operating in
TR mode according to the preferred embodiments.
[0097] In this regard, and in contrast to FIG. 2, FIG. 2A
illustrates the response of the AFM when operating the AFM in TR
mode. The difference in the response to tip-sample interaction is
dramatic. For the same change in tip-sample separation, .DELTA.Z, a
much larger change in the detected voltage signal, .DELTA.V.sub.t,
is observed. Because .DELTA.V.sub.t/.DELTA.Z is much larger than
the slope associated with flexural mode, .DELTA.V.sub.f/.DELTA.Z, a
significant improvement in imaging sensitivity and data acquisition
speed is realized. As a result, more accurate images (i.e., small
changes in tip-sample interaction are readily detected) can be
obtained at greater speeds.
[0098] One aspect of driving the probe into torsion in addition to
that described above is fabricating the probe to improve the
inertial effects of the probe itself on the corresponding
oscillation. In particular, the construction of the probe may be
intentionally modified to improve the efficiency of achieving
torsional oscillation. In this regard, the probe may be made
asymmetric along a longitudinal centerline of the probe, thus
facilitating oscillation at, for example, a torsional resonance of
the lever. This is accomplished using a microfabrication process to
place the tip off the centerline of the probe, a process that is
relatively easy to control. Alternatively, or in addition, the tip
height may be made greater than the tip height of conventional
probes, thus facilitating excitation of the probe to produce
torsional motion thereof.
[0099] For examplary purposes only, we refer to FIG. 15, showing
each of these features in phantom. In particular, a tip 309 having
a larger tip height than the nominal tip height of conventional tip
307 is shown optionally in phantom. Notably, a conventional tip 307
has a nominal tip height of about 10-15 .mu.m. Preferably, this
larger tip height is greater than 15 .mu.m, and more preferably
about 20 .mu.m. FIG. 15 also shows a cantilever 311 of the probe in
phantom in which tip 307 (or 309) is offset from the centerline of
the cantilever 311, thus providing the asymmetry and inertial
benefits previously mentioned.
[0100] A similar characteristic of the probe that may be modified
to facilitate exciting torsional oscillation is fabricating the
cantilever itself so that its shape adds to the inertial force
responsible for creating the torsional motion. More particularly,
by making the cantilever asymmetrical, for example, by placing a
paddle-shaped portion having an asymmetric shape near its end (see
315 in phantom in FIG. 15), the difference in mass on either side
of the centerline of the cantilever takes advantage of inertial
forces to facilitate efficiently oscillating the cantilever in
torsion. All in all, rather than the previous embodiments that
specifically align the axis of rotation of the drive with the tip,
torsional oscillation with each of these probe construction
characteristics can be effectively enhanced and/or excited using a
conventional drive.
[0101] Although the best mode contemplated by the inventors of
carrying out the present invention is disclosed above, practice of
the present invention is not limited thereto. For example, the
sample may be subjected to alternate environments (e.g., varied
media, atmospheric conditions, etc.), and the corresponding
torsional mode response analyzed. Notably, such alterations may
change oscillation properties of the probe as desired by the user.
Moreover, a voltage phase control unit may be used in addition to
the mode control unit to modify the phase of the two drive signals
in the preferred embodiment, to selectively operate the AFM in
torsional and flexural modes. It will be manifest that various
additions, modifications and rearrangements of the features of the
present invention may be made without deviating from the spirit and
scope of the underlying inventive concept.
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